Lebanon's Hezbollah-backed prime minister-designate has held talks with political leaders to begin forming a new government, as differences over a United Nations tribunal investigating the 2005 assassination of former prime minister Rafiq Hariri threaten that effort.

The movement of outgoing Lebanese Prime Minister Saad Hariri asked the new incoming prime minister, Najib Mikati, Thursday whether he will continue ties to the U.N. tribunal investigating the death of the former prime minister, Mr. Hariri's father.

Saad Hariri's political ally, Fuad Siniora, says the movement has submitted a list of demands to Mr. Mikati to clarify his position on the tribunal.

The U.N. tribunal's investigation is at the center of Lebanon's political crisis.

The Shi'ite militant group Hezbollah wants Lebanon to cut ties with the tribunal and denies any role in the killing of Mr. Hariri's father. Hezbollah and its allies resigned from Saad Hariri's Western-backed coalition Cabinet, causing it to collapse earlier this month, when he refused to denounce the tribunal.

The tribunal's still-secret indictment issued last week is expected to accuse Hezbollah members of involvement in the 2005 assassination.

U.S. State Department spokesman P.J. Crowley said Thursday the U.S. will judge the new Lebanese government by its actions. He said "a clear test" will be the government's willingness to continue to support the tribunal's work.

Mr. Mikati said he would form a Cabinet of technocrats if the party of the outgoing prime minister refuses to join the government.

Mr. Mikati has said he wants to include all parties in his Cabinet, but Mr. Hariri has said he will not join a government led by someone picked by Hezbollah.

In neighboring Syria, one of Hezbollah's foreign patrons, the Syrian foreign minister Walid al-Muallem on Thursday urged all sides to join a unity government.

The United States is reconsidering economic and military support for Lebanon after Hezbollah won a prominent role in the government of the fragile, religiously divided nation.

Thousands of Mr. Hariri's angry supporters took to the streets Tuesday in several cities, where they shouted their loyalty to the former leader. Some protesters said they would not allow Lebanon to go down "an Iranian path," a reference to Tehran's support for Hezbollah.

Some information for this report was provided by AP, AFP and Reuters.

NEW: Follow our Middle East reports on Twitter and discuss them on our Facebook page.